A general ultrasonic sensor provides a unimorph structure by bonding a piezoelectric element to an inner surface of a bottom portion of a case made of metal and transmits and receives ultrasonic waves by causing the bottom portion of the metal case to perform bending vibration. Japanese Unexamined Patent Application Publication No. 2002-204497 (Patent Document 1) discloses an ultrasonic sensor including a multilayer piezoelectric element.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-204497.
When, for example, the ultrasonic sensor as disclosed in Japanese Unexamined Patent Application Publication No. 2002-204497 (Patent Document 1) is driven, the following characteristics are provided. That is, as the number of laminated layers of the piezoelectric element included in the ultrasonic sensor increases, sound pressure in transmission is increased whereas sensitivity in reception is decreased. By contrast, when a piezoelectric element in which the number of laminated layers is reduced as disclosed in the above-described document, the sound pressure in the transmission is decreased whereas the sensitivity in the reception is increased.
Thus, in the conventional multilayer piezoelectric element used for the ultrasonic sensor, a portion of the piezoelectric element which is provided for the transmission and a portion thereof which is provided for the reception, are formed in the same section. Therefore, the conventional multilayer piezoelectric element provides characteristics in which the sound pressure and the sensitivity are increased and decreased, respectively, being substantially proportional or inversely proportional to the number of laminated layers of the piezoelectric element. The ultrasonic sensor including the conventional multilayer piezoelectric element has been under an actual circumstance that it is difficult to adjust both the sound pressure in the transmission and the sensitivity in the reception independently of each other.